1. Field of the Invention
The present invention relates to an electromagnetic actuator and an optical disk apparatus incorporating the same and, more particularly, to a linear actuator for driving an optical head incorporated in an optical disk apparatus for reproducing/recording information from/in an optical recording medium by medium.
2. Description of the Related Art
An electro-magnetic actuator as an actuator for driving a head is currently widely used in various fields of, e.g., an optical disk apparatus and a magnetic disk apparatus. For example, as an electromagnetic actuator for driving an optical head used in an optical disk apparatus, a mechanism as shown in FIG. 1 is known. In the mechanism shown in FIG. 1, an objective lens 102 for focusing a light beam from a light source onto an optical disk 101 is mounted on a movable member 103. The objective lens 102 can be finely moved by a short distance along its optical axis toward the optical disk 101 and can be also finely moved by a short distance in the radial direction of the optical disk 101 by a mechanism provided in a cover 104 to move the objective lens. A tracking driving mechanism for driving the objective lens 102 in the radial direction of the optical disk 101 by a comparatively long distance comprises a pair of magnetic circuits 107 each constituted by a yoke 105 and a permanent magnet 106 fixed to the yoke 105, a tracking coil 108 fixed to the movable member 103, linear guides 109 for regulating a moving direction of the movable member 103, and guide rollers 110 which can roll on the linear guides 109. The movable member 103 is roughly driven in the radial direction of the optical disk 101 by a Lorentz force generated by a current flowing through the tracking coil 108 and magnetic fluxes flowing in the magnetic circuits 107, and the objective lens 102 is finely driven by the above objective lens driving mechanism to form a light beam spot at a desired position on the surface of the optical disk 101, thereby recording or reproducing information in or from the optical disk 101.
As shown in FIG. 2, in the magnetic circuit 107 to be incorporated in such a mechanism, a plate-like yoke 105a and a yoke 105b having a substantially U-shaped section are connected to form the yoke 105, and the permanent magnet 106 is fixed on the inner surface of the yoke 105a and arranged in a space between the yokes 105a and 105b.
In the magnetic circuit 107 having the above arrangement, as shown in FIG. 3A, a magnetic path is formed such that not all of magnetic fluxes generated by the N pole of the permanent magnet 106 flow straight to the longitudinal section of the yoke 105b but some magnetic fluxes flow toward two short sides of the yoke 105b. In addition, as shown in FIG. 3B, magnetic fluxes generated by the end portion of the permanent magnet 106 form a magnetic path which returns directly to the yoke 105a through air outside the magnetic circuit. Therefore, a magnetic flux density distribution in a magnetic gap is not uniform along the inner surface of the yoke 105b but a magnetic flux density at the end portion becomes smaller than that at the center of the magnetic gap.
When the size of the magnetic circuit 107 is decreased, it becomes very difficult to maintain a uniform magnetic flux density distribution in the magnetic gap along the inner surface of the yoke 105a due to magnetic characteristics of the permanent magnet 106 or material characteristics such as magnetic saturation of the yoke 105. As a result, a considerably large amount of magnetic fluxes leak into air having a smaller permeability than that of the yoke 105. When a magnetic path is formed in this manner, magnetic fluxes at the center of the magnetic gap largely differ from those at its end portion. Therefore, if such a magnetic circuit is applied to the electro-magnetic actuator shown in FIG. 1, a generated driving force changes in accordance with the position of the movable member 103 and degrades uniformity of moving acceleration of the objective lens 102. As a result, positioning control of the objective lens 102 becomes unstable.
As described above, a magnetic path formed by the conventional magnetic circuit is not uniform along the inner surface of the magnetic gap, and it is very difficult to maintain uniformity of a magnetic flux density distribution in the magnetic gap if the magnetic circuit is made smaller in size. Therefore, if this magnetic circuit is applied to an electro-magnetic actuator, a generated driving force changes in accordance with the position of a movable member, resulting in unstable positioning control of an objective lens.
An optical system of an optical head of the mechanism shown in FIG. 1 generally comprises three systems. i.e., a guiding optical system for guiding a light beam from a light source, an optical pick-up system for focusing the light beam onto an optical disk and picking up the light beam from the optical disk, and a detecting system for detecting the light beam. More specifically, in the guiding optical system, a light beam emitted from a light source, e.g., a semiconductor laser is shaped and collimated. In the optical pick-up system, the optical pick-up system, the light beam transmitted from the guiding optical system is focused on a rotating optical disk by the objective lens 102. In the detecting system, the light beam modulated and reflected by a recording surface of the disk is focused on a photodetector and detected for signal reading and position detection. In the conventional optical head shown in FIG. 1, the guiding optical system and the detecting system having a considerably large total weight (generally 50 g or more) are mounted on the chassis 103 as a movable member. Therefore, in order to drive the optical head at a high speed, the magnetic circuit 107 capable of generating high power, i.e., the magnetic circuit 107 capable of generating high power, i.e., the magnetic circuit 107 having a comparatively large size is required. In this case, since the size of the movable member is naturally increased, further limitations are imposed if the optical head must be housed and operated in a limited space in the optical disk apparatus.
Recently, a separate type optical head device shown in FIG. 4 in which a guiding optical system and a detecting system are separated from a movable member to reduce the weight of the movable member and realize high-speed driving has been used. In this separate optical head device, a lens bobbin 202 to which an objective lens 102 is connected is elastically supported by parallel leaf springs 204, thereby supporting the objective lens 102 to move parallel to its optical axis direction (Z direction). A flat type focusing coil 206 as a focusing driving system is wound around the side surface of the lens bobbin 202 to have its axis in the Y direction. The focusing coil 206 and a fixed magnetic circuit (not shown) form an electro-magnetic driving system in which a Lorentz force, i.e., a driving force for driving the objective lens 102 in the Z direction is generated in accordance with a Fleming's left-hand rule by the direction (X direction) of a current flowing through the focusing coil 206 and the direction (Y direction) of magnetic fluxes generated by a permanent magnet of the magnetic circuit. The lens bobbin 202 is moved by this driving force while equally curving the two leaf springs. That is, the objective lens 102 is driven in the optical axis direction. A reflecting mirror 240 is arranged below the objective lens 102 to deflect a light beam, emitted from a light source and passed through a fixed guiding optical system, through 90.degree. and radiate the deflected light beam onto a disk 101. A light beam modulated and reflected by the upper surface of the disk 101 is guided to a detecting optical system by the reflecting mirror 240.
Guide rollers 110 such as bearings elastically supported by support pins 210 are arranged at both side surfaces of a carriage 208 as a movable member and roll along linear guides 109 with a circular section fixed on a base (not shown) and elongated in the X direction. Therefore, the carriage 208 is moved in the disk radial direction (X direction) while its two ends are supported by the linear guides 109. A tracking coil 214 is wound around a tracking coil bobbin portion 212 of the carriage 208 to have its axis in the X direction. The tracking coil 214 is inserted in a non-contacting state into a magnetic gap between a yoke and the permanent magnet of the magnetic circuit and forms a voice coil motor together with the magnetic circuit. This voice coil motor generates a Lorentz force, i.e., a driving force for driving the carriage 208 in the X direction by the tracking coil 214 and the direction (Z direction) of magnetic fluxes in the magnetic gap of the magnetic circuit. The carriage 208 is moved by this driving force while the rollers 110 roll on the linear guides 109. That is, the objective lens 102 is moved in the X direction.
Of the two conventional optical heads described above, in the optical head device shown in FIG. 1 in which both the guiding and detecting optical systems are mounted on the movable member 103, it is difficult to record/reproduce information at a high speed since reduction in weight of the movable member is limited.
In the separate type optical head device, in order to improve response characteristics upon X-direction driving of the movable member 208, it is preferred to sufficiently narrow the magnetic gap, i.e., move the tracking coil 214 inserted in the magnetic gap as close as possible to the yoke surface or the magnet surface opposing each other above and below the tracking coil 214. In order to allow the guide roller 110 to rotate on the linear guide 109 with a low frictional resistance, the rotating shaft of the guide roller and the linear guide direction must be set to be substantially perpendicular to each other. In addition, in order to restrict the optical head in a direction except for the tracking direction by a plurality of guide rollers 110, an inclination and a positional accuracy of each pin 210 for supporting the guide roller 110 must be set within predetermined allowable ranges. Since, however, mounting accuracies of the tracking coil 214 and the movable member 208 are actually not so high, it becomes difficult to maintain a non-contacting state between the tracking coil 214 and the yoke if the magnetic gap is made narrower. Similarly, since mounting accuracies of each pin 210 and the movable member 208 are not so high, it is difficult to set the inclination and positional accuracy of each pin 210 within allowable ranges.
In order to stabilize optical characteristics and Z-direction driving of the objective lens 102, a high mounting accuracy and inclination accuracy are required for the lens bobbin 202 or the focusing coil 206. However, a demand for a high mounting accuracy of the focusing coil 206, the tracking coil 214, the lens bobbin 202, the pin 210, the reflecting mirror 240, and the like, and a complicated shape of the separate optical head cause reduction in productivity in a manufacturing/assembling process of the optical head. The above demand for a high mounting accuracy of the pin 210, the lens bobbin 202, the focusing coil 206, and the like in the separate type optical head device is similarly present for a mounting accuracy between the movable member 103 and the guide roller support pin and between the movable member 103 and the optical pick-up 104 in the standard optical head device described above.
In addition, in order to realize an optical head which can perform recording/reproduction at a high speed in a future, an optical head in which a movable member is arranged so as not to produce an unnecessary vibration in a moving optical element and which can be driven without an inclination or offset of an optical axis by eliminating an unnecessary mass distribution must be developed.